Magnetic random access memories (MRAM), which are expected to serve as a nonvolatile memory that can operate at high speed and perform an infinite number of times of rewriting, have been intensively developed. In an MRAM, magnetic material is used as a memory element, and information is memorized as the direction of magnetization of the magnetic material. Several methods are proposed as a method for switching the magnetization of the magnetic material, and all of them are common in use of a current. To put an MRAM into practical use, the reduction in the write current is of much significance; according to 2006 Symposium on VLSI Circuits, Digest of Technical Papers, p. 136, the reduction down to 0.5 mA or less, more preferably, the reduction down to 0.2 mA or less is required.
One of the most popular methods for writing information into the MRAM is a method that arranges an interconnection line used for writing around a magnetic memory element, and switches the direction of magnetization of a magnetic memory element with use of a magnetic field generated by flowing a current through the interconnection line. This method achieves magnetization reversal by the magnetic field, and can therefore, in principle, perform writing in one nanosecond or less; this is preferable for achieving a high speed MRAM. However, a magnetic field for switching magnetization of magnetic material with sufficient thermal stability and resistance against a disturbance magnetic field is typically around several tens of oersteds, and a current of approximately several mili-amperes is required to generate such a magnetic field. In this case, the chip area is inevitably increased, and power consumption necessary for writing is also increased; this results in that the MRAM is inferior to the other random access memories in competitiveness. In addition, when the element is miniaturized, the write current is further increased, which is not preferable also in scaling.
In recent years, the following two approaches are proposed as a method for solving such problems. The first approach is to use spin injection magnetization reversal. This is a method that, in a film stack including a first magnetic layer having reversible magnetization, and a second magnetic layer electrically connected to the first magnetic layer and having fixed magnetization, reverses the magnetization of the first magnetic layer by using the interaction between spin-polarized conduction electrons and localized electrons in the first magnetic layer in the case where a current is flowed between the first and second magnetic layers. The spin injection magnetization reversal occurs at a certain current density or more, and therefore, the current necessary for writing is reduced as the element size is decreased. In other words, the spin injection magnetization reversal method is superior in terms of scaling. In general, however, an insulating layer is provided between the first and second magnetic layers, and a relatively large current should be flowed through the insulating layer in writing; this causes a problem in rewriting resistance or reliability. Also, in general, the write current path and the read current path are same, and therefore undesired writing in reading is also concerned. That is, it can be said that the spin injection magnetization reversal is superior in terms of scaling, but has some barriers for practical application.
The other method is to use the current-driven domain wall motion phenomenon. The magnetization reversal method using the current-driven domain wall motion phenomenon can solve the above-described problems associated with the spin injection magnetization reversal. An MRAM using the current-driven domain wall motion phenomenon is disclosed in, for example, Japanese Laid Open Patent Applications No. P2005-123617A, P2005-191032, P2006-73930A, P2006-270069A, and P2006-287081A. In particular, Japanese Laid Open Patent Application No. P2006-73930A discloses a magnetoresistance effect element including magnetic films each having a magnetization in the thickness direction.
In a typical MRAM using the current-driven domain wall motion phenomenon, magnetizations at both ends of a first magnetic layer having reversible magnetization are fixed so as to be substantially antiparallel to each other. In such magnetization arrangement, a domain wall is introduced in the first magnetic layer. It should be noted that, as reported in Physical Review Letters, vol. 92, No. 7, 077205 (2004), the domain wall moves toward conduction electrons when a current is flowed in the direction in which the current passes through the domain wall, and therefore a writing is achieved by flowing a current through the first magnetic layer. The current-driven domain wall motion also occurs at a certain current density or more, and therefore this method is superior in scaling as is the case of the spin injection magnetization reversal. In addition, it would be understood that the MRAM element using the current-driven domain wall motion phenomenon solves the above problems as described for the spin injection magnetization reversal, since the write current does not flow through the insulating layer, and the write current path and the read current path are separated.
It is concerned, however, that the absolute value of the write current may be relatively large in the MRAM using the current-driven domain wall motion phenomenon. A large number of reports on the observation of current-induced domain wall motion are made besides Physical Review Letters, vol. 92, No. 7, 077205 (2004), and inmost of them, a current density of approximately 1×108 [A/cm2] is required for domain wall motion. In such a case, the write current is 1 mA for the case where the width and film thickness of the layer in which the domain wall motion occurs are respectively assumed to be 100 nm and 10 nm, for example. The reduction of the write current down to this value or less may be achieved by decreasing the film thickness; however, it is known that the current density necessary for writing is further increased as the film thickness is decreased (see, for example, Japanese Journal of Applied Physics, vol. 45, No. 5A, pp. 3850-3853, (2006)).
Also, to induce the current-driven domain wall motion, a width of the layer in which the domain wall motion occurs should be reduced to a few 10 s nm or less; however, this involves a large difficulty in fabrication technique.
In addition, influences of electron migration and temperature rising may be issues, regarding the use of the current density close to 1×108 [A/cm2] for the domain wall motion.